Typically, in performing a spray operation, such as a thermal spray operation, a component or part (such as a medical implant component) is placed on a holding fixture which is operable to rotate. Examples of a thermal spray operation may include a plasma spray operation, a high velocity oxygen fuel (HVOF) spray operation, and so forth. While the holding fixture and the medical implant component rotate, a desired material is sprayed onto the outer surface of the medical implant component so as to form a coating layer by use of a spray gun, such as a thermal or plasma type spray gun.
One type of holding fixture holds a single medical implant component in the center thereof. During a spray operation, such type of fixture rotates causing the medical implant component to also rotate. Another type of holding fixture may hold a plurality of medical implant components. In such other type of fixture, the medical implant components may be moved or rotated so that a respective medical implant component may be indexed into a spray position adjacent to the spray gun. Thereafter, the fixture may cause the respective medical implant component to be rotated about a respective axis, while such medical implant component is sprayed. During such rotation and spraying of the respective medical implant component, the other medical implant components are kept stationary. In other words, in this type of holding fixture, all of the medical implant components may be moved or rotated while one such component is being indexed, but when the respective medical implant component is rotated and being sprayed the other medical implant components do not move or rotate.
In a thermal or plasma spraying operation, the particles of the spray material may be heated to a relatively high temperature (such as 0.7 to 0.9 of its melting point or even at or higher than its melting point). For example, if the spray material is chromium oxide (Cr2O3), it may be heated to a temperature higher than its melting point of approximately 2450 degrees Centigrade during such thermal spraying operation. As a result, the temperature of the coating layer of the medical implant component may be relatively high. Although the rotation of the medical implant component on the holding fixture during the spraying process may help to cool most of the outer surface or coating layer of the implant component by convection, at least one part thereof may not be cooled due to such rotation. More specifically, if a medical implant component, such as a symmetrically shaped femoral head, is placed on either of the types of holding fixtures previously described, the top center of the femoral head does not move while being rotated during the spray process. Instead, during such rotation, the top center of the femoral head remains in the same location. Accordingly, since the top center of the medical implant component does not move during the spray process, such portion may not be cooled by convection. As a result, localized over-heating may occur which, in turn, may cause cracking of the coating layer of the medical implant component.
To minimize over-heating, the spraying could be stopped or interrupted after each pass so as to allow the coating layer to cool. Although this method may minimize or reduce over-heating of the coating layer during a spraying process, such method may increase the cost of the spraying operation. That is, if the coating layer is allowed to cool between each pass, the time or duration of the spraying process is increased. Such increased time may result in increased cost.
In addition to localized high temperatures, other factors may also cause cracking in the coating layer. More specifically, cracking in the coating layer may occur when the residual stress (σR) is greater than the coating strength. The residual stress (σR) may be equal to:σR=E(αc−αm)(ΔT)ƒ1(coating layer thickness)ƒ2(shape)ƒ3(thermal conductivity)  (Eq. 1)in which E is the modulus of elasticity of the coating material, αc is the coefficient of thermal expansion of the coating material, αm is the coefficient of thermal expansion of the material or metal of the substrate of the medical implant component, ΔT is the difference between room temperature and a pre-heat temperature of the substrate during the spray operation, ƒ1 (coating layer thickness) is a function relating to the total thickness of the coating layer and/or the thickness of the coating material applied per pass, ƒ2 (shape) is a function relating to the shape of the component, and ƒ3 (thermal conductivity) is a function relating to the thermal conductivity of the substrate material and/or the coating material. As a result, one or more factors such as the difference between room temperature and the pre-heat temperature of the substrate during the spray operation, the total thickness of the coating layer, the thickness of the coating material applied per pass, the shape of the component (e.g., whether the component has a sharp corner or a curved surface with a relatively large or small radius), the value(s) of the thermal conductivity of the substrate material and/or the coating material, and the difference in the thermal coefficient of expansion for the coating material and that of the substrate material, may cause cracks to develop in the coating layer. For example, cracking of the coating layer may occur if too much spray material is applied within a pass or within a given time interval.
Accordingly, to avoid cracks from occurring in the coating layer a number of parameters may be followed. For example, (i) the materials for the coating layer and the substrate may be selected such that the difference in the thermal coefficients of expansion of such materials is less than a predetermined value, such as less than approximately 1.0×10−6/degree Centigrade, and such that the thermal conductivity thereof have acceptable values, (ii) the component or the surfaces thereof to be sprayed may be designed such that sharp corners are avoided and curved surfaces have a relatively large diameter, such as equal to or greater than approximately 42 millimeters, (iii) the total thickness of the sprayed material and/or the thickness of material sprayed or applied during each pass may be less than a predetermined value, and (iv) the temperature of the substrate and/or coating layer may be controlled such that the difference in temperature (ΔT) of the substrate during the spray process may be maintained so as not to exceed a predetermined value.
As is to be appreciated, it may be difficult to vary the elements of items (i) and (ii) above so as to provide the most acceptable situation. That is, the shape of the component (along with the surface or surfaces thereof to be coated) may be substantially fixed due to the actual size of the bones and so forth of a patient; and, the materials for the substrate and the coating layer may be selected so as to satisfy other objectives, such as long term wear, biocompatibility, lack of noise during use, and so forth. However, the elements of items (iii) and (iv) may be more easily varied to obtain an acceptable situation.
As such, it would be advantageous to provide a system which would enable components (such as medical implant components) to be sprayed with a desired material by a thermal or plasma type spraying process or the like which would control the amount of materials which are sprayed such that the total thickness of such material applied and/or the thickness of such material applied per pass does not exceed a predetermined value, and which would maintain the pre-heat temperature and/or the difference in temperature (ΔT). Additionally, it would also be advantageous to provide such system which would operate in a cost efficient manner.